There is a difference in intended results between purposefully over-exposing print film vs shooting it at a speed higher than the manufacturers rating. I base print film speed on the amount of exposure required to build a midtone density that hits a reference film supplied by the manufacturer. Using the toe/heel method will drive you nuts and isn't practical with print films because of the different ways that Fuji and Kodak emulsions handle increasing density.
Notorious examples of print films that don't work well at their rates speed are NPH, VPS and to some extent older Reala.
Print films that do work well at their rated speed are Kodak Supra 100/400 and Kodak Gold 100. Most of the amatuer films handle well at their rated speeds, expcuding Superia-Reala which tends to prefer being rated at slower than EI 100.
Example: Shoot a grey card with Gold 100, and then NPH exactly two stops down. Now measure the density of those frames while subtracting the difference in the clear base. You'll find that NPH IS NOT two stops faster than Gold 100 and NPH will correspondingly suffer if shot at EI 400.
The other side of the coin is simply over-exposing print films to alter their inherent picture qualities. 10 years I might have agreed that halving the ASA of any print film will lead to better images. With current emulsion technology that's not the case, and too much over-exposure can get you into trouble, especially with higher contrast Kodak films like Supra. Fuji print films by nature of their built in compensation technology typically like a full stop of over-exposure. Kodak print films like the Portra series work well rated about 1/3 slower that their listed speed, but they gain less advantage than the Fuji print films in terms of deliberate over-exposure.
So, what's my point: The problem with rating all print films 1/2 of their rated speed is that doing so may result in several frames of random shots on that roll ending up much more over-exposed than a single stop. A stop of over-exposure is no big deal for print film, and even two stops. The problem is that a frame of print film two stops over-exposed is going to be a bitch to scan later on and throw off the color balance on any automated printer. Guaranteed that shots with proper exposure will have a different print color balance than shots one or two stops over. Most commercial printers can barely slope out a one stop bracket either way in terms of keeping linear color balance.
Increasing exposure with print film results in increasing saturation of the dye-clouds in the emulsion, and the net result is a "smoother", and in some cases more saturated color. The drawback is that you'll start to lose detail and sharpness because of halation effects, dye-coupler crossover, and other strange things that happen with more over-exposure. NHG I and Agfa Ultra 50 are two films that could not be safely over-exposed without some obvious loss of image quality.
The best way to get around all these variables is to shoot a test roll of any print film at various speeds, take notes of those speeds, and compare the proofs when you get them from the lab. Note where your proofs tend to "even in out" in terms of quality and this will be a good personal EI for that print film.
Photo by Yannick J.
